Incomplete dominance:
- When a cross between two pure breed is done for one contrasting character, the F1 hybrid phenotype dose not resemble either of the two parents and was in between the two, called incomplete dominance.
- Inheritance of flower color in the dog flower (snapdragon or Antirrhinum sp.) is a good example of incomplete dominance.
- F2 generation phenotypic ratio is 1:2:1 in stead of 3:1 as Mendelian monohybrid cross.
- Genotypic ratio of F2 generation is 1:2:1.
Co – dominance:
- F1 resembled either of the two parents (complete dominance).
- F1 offspring was in-between of two parents (incomplete dominance).
- F1 generation resembles both parents side by side is called (co-dominance).
- Best example of co-dominance is the ABO blood grouping in human.
- ABO blood group is controlled by the gene I.
- The plasma membrane of the RBC has sugar polymers (antigen) that protrude from its surface and the kind of sugar is controlled by the gene-I.
- The gene I has three alleles I A,I B and i.
- The alleles I A and I B produce a slightly different form of sugar while allele i doesn’t produce any sugar.
- Each person possesses any two of the three I gene alleles.
- I A and I B are completely dominant over i.
- When I A, and I B present together they both express their own types of sugar; this because of co-dominance. Hence red blood cells have both A and B type sugars.
Multiple Alleles:
- Example of ABO blood grouping produces a good example of multiple alleles.
- There are more than two i.e. three allele, governing the same character.
A single gene product may produce more than one effect:
- Starch synthesis in pea seeds is controlled by one gene.
- It has two alleles B and b.
- Starch is synthesized effectively by BB homozygote and therefore, large starch grains are produced.
- The ‘bb’ homozygous has less efficiency hence produce smaller grains.
- After maturation of the seeds, BB seeds are round and the bb seeds are wrinkle.
- Heterozygous (Bb) produce round seed and so B seems to be dominant allele, but the starch grains produced are of intermediate size.
- If starch grain size is considered as the phenotype, then from this angle the alleles show incomplete dominance.
INHERITANCE OF TWO GENES:
Law of independent Assortment:
- When two characters (dihybrid) are combined in a hybrid, segregation of one pair of traits is independent of the other pair of traits.
CHROMOSOMAL THEORY OF INHERITANCE:
Why Mendel’s theory was remained unrecognized?
- Firstly communication was not easy in those days and his work could not be widely publicized.
- Secondly his concept of genes (or factors, in Mendel’s word) as stable and discrete units that controlled the expression of traits and of the pair of alleles which did not’ blend’ with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature.
- Thirdly Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
- Finally he could not provide any physical proof for the existence of factors.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
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